In the galvanizing of wire by applying a zinc coating to the surface thereof or in other coating operations, it is necessary for the wire surface to be free from oxides and other impurities to obtain satisfactory adhesion of the zinc or other coating material. It is conventional to treat the iron wire after annealing and before galvanizing with hot hydrochloric acid, sulphuric acid or other acid to remove surface impurities, such as scale, grease and drawing compound, in an operation conventionally termed "pickling".
In a conventional pickling operation, a plurality of wires is drawn through a static open bath of heated hydrochloric acid, before being washed, fluxed and galvanized in line. Problems arise with this prior art procedure. The open nature of the bath and the requirement for elevated temperature operation result in hydrochloric acid fumes, which must be removed via fume hoods and vented from the working area to atmosphere, which gives rise to significant environmental pollution. The venting is never perfect, however, and working area contamination results.
In addition, the wires must be maintained submerged in the bath for the pickling to be effective. This requires the wires to be guided to below the surface of the bath at one end of the bath and then to be guided out from below the surface at the other end of the bath. These changes in direction apply stress to the wire, which can cause kinking of the wire, imposes limitations on the size of wire which can be drawn through the bath, and increases the energy required to draw the wire through the bath. Further, this arrangement provides a rethreading problem upon breakage of a wire.
The static nature of the operation relies on movement of the wire through the bath to remove the impurities. Varying conditions often exist in varying parts of the bath, leading to inconsistent pickling. The bath also must be replenished and replaced from time to time, since the effectiveness of the bath diminishes with time as the acid is depleted. Such replenishment is a difficult and time consuming operation, which causes significant downtime of the galvanizing time.